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Insect Salivary Proteins Prevent Blood
Clotting

Every summer, both people and livestock face pesky mosquitoes, gnats, and
other biting flies.

The resultant itching and discomfort come from the immune system reacting to
proteins in the insects' salivary glands. These proteins help victims' blood to
flow so the insects can feed easier. They may also aid the transmission of
insect-borne pathogens that can cause diseases.

Scientists at the Agricultural Research
Service Arthropod-borne Animal Diseases Research Laboratory in Laramie,
Wyoming, have identified several such proteins in a small biting midge called
Culicoides variipennis.

This pest is the principal carrier of the viruses that cause bluetongue in
North American cattle. When an infected midge feeds, it can simultaneously
release the viruses into the animal's bloodstream. Bluetongue costs $100
million in lost trade annually, because countries without the disease won't
accept some U.S. livestock exports.

ARS entomologists Walter J. Tabachnick and Adalberto Perez de Leon have
found that the midges secrete several proteins during feeding.

"Some of the proteins cause blood vessel dilation, another acts as an
anticlotting agent, and one inhibits blood platelets from aggregating and
helping in clotting," Tabachnick says.

Still other salivary proteins inhibit pathogen-fighting cells like
macrophages and lymphocytes, weakening an animal's ability to fight off the
viruses. "The insect creates a very conducive environment in the
host," says Tabachnick.

The next step, he says, is to develop vaccines against these insect
proteins, to reduce the spread of bluetongue viruses.

Surprisingly, though, there may also be some beneficial uses for the
proteins. "We see great potential that these proteins might serve as
pharmaceuticals for humans and livestock--such as anticlotting factors,
vasodilators to widen the inner bore of blood vessels, or immunosuppressives to
depress natural immune responses," says Tabachnick.

The researchers are also studying the role of insect salivary gland proteins
from biting flies in spreading another important livestock disease, vesicular
stomatitis. Preliminary results indicate that, like the midges, other biting
flies release substances into an animal's blood that reduce its ability to
fight off the disease-causing virus.--By Dennis Senft, formerly with
ARS.